Fabrication of Cu–Zr–Ti thick film metallic glass structure by double metal mask lift-off process

Abstract

In micro-electro-mechanical systems (MEMS) technology, one of the conventional patterning methods used for metallic thin films is the lift-off process using a photoresist material. However, there is concern regarding the photoresist material generating gas due to the long-term annealing or the high power used in sputter deposition. Thin-film amorphous alloys, such as thin-film metallic glass (TFMG), when used in the device, become crystallized by such heating and outgassing during sputtering. Unlike photoresist material, a metal mask does not generate gas during sputtering. However, a metal mask is subject to deformation by the stress in the sputtered film on the mask. The pattern shape of the TFMG structure can thus collapse due to this mask deformation. Its cross-sectional shape then deviates significantly from a rectangle. We propose a novel lift-off process using a double metal mask for reducing changes in the cross-sectional shape, and maintaining the amorphous nature of the TFMG structure. The characteristic of the double metal mask lift-off process is duplication of the metal mask for restraint of the lower metal mask deformation. Since nearly all of the sputtered material is deposited on the upper metal mask, the material is only minimally deposited onto the lower metal mask. In this work, the double metal mask lift-off process was compared with the conventional lift-off process, and important points for using the double metal mask lift-off process were elucidated. Using the double metal mask lift-off process, a very thick amorphous TFMG structure could be obtained, as compared to the conventional lift-off process. To maintain the rectangular cross-sectional shape of the structure, it was found to be necessary to decrease the height of the double metal mask, as compared to the conventional lift-off process, in anticipation of upper metal mask deformation.